Cement compositions comprising humic acid grafted fluid loss control additives

ABSTRACT

Exemplary embodiments of the cement compositions comprise water, a cement, and a fluid loss control additive, the fluid loss control additive comprising a graft copolymer. The graft copolymer comprises a backbone and at least four grafted monomers selected from the group consisting of 2-acrylamido-2-methylpropanesulfonic acid, acrylamide, acrylic acid, vinylphosphonic acid, diallyldimethylammonium chloride, and salts thereof. The backbone of the graft copolymer comprises a humic acid salt. Methods of cementing using exemplary embodiments of the cement compositions are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. ______ entitled “Methods of Using Humic Acid Grafted Fluid Loss Control Additives in Cementing Operations” (Attorney Docket No. 2006-IP-022148U1), filed on the same date herewith, the entire disclosure of which is incorporated herein by reference.

BACKGROUND

The present invention relates to cementing operations. More particularly, the present invention relates to cement compositions comprising a graft copolymer fluid loss control additive, the graft copolymer fluid loss control additive comprising a backbone of a humic acid salt and at least four grafted monomers selected from the group consisting of 2-acrylamido-2-methylpropanesulfonic acid, acrylamide, acrylic acid, vinylphosphonic acid, diallyldimethylammonium chloride, and salts thereof.

Well cement compositions are commonly utilized in subterranean operations, particularly subterranean well construction and remedial operations. For example, in subterranean well construction, a pipe string (e.g., casing and liners) may be run into a well bore and cemented in place. The process of cementing the pipe string in place is commonly referred to as “primary cementing.” In a typical primary cementing operation, a cement composition may be pumped into an annulus between the walls of the well bore and the exterior surface of the pipe string disposed therein. The cement composition sets in the annular space, thereby forming an annular sheath of hardened, substantially impermeable cement that supports and positions the pipe string in the well bore and bonds the exterior surface of the pipe string to the subterranean formation. Among other things, the annular sheath of set cement surrounding the pipe string functions to prevent the migration of fluids in the annulus, as well as protecting the pipe string from corrosion. Cement compositions also may be used in plugging and abandonment operations and remedial cementing operations, such as squeeze cementing and the placement of cement plugs.

For such subterranean cementing operations to be successful, the cement compositions typically include a fluid loss control additive to reduce the loss of fluid, from the cement compositions, e.g., when they contact permeable subterranean formations and zones. Excessive fluid loss may cause, inter alia, a cement composition to become prematurely dehydrated, which limits the amount of the cement composition that can be pumped. Due to this premature dehydration, excessive pump pressure may be required to place the cement composition, potentially resulting in breakdown of the formation and/or destabilization of the well bore.

Certain polymers have been used heretofore as fluid loss control additives in cementing operations. For example, cellulosic materials such as hydroxyethyl cellulose and carboxymethylhydroxyethyl cellulose have been used as fluid loss control additives. Copolymers of 2-acrylamido-2-methylpropanesulfonic acid and acrylamide or N,N-dimethylacrylamide have also been used as fluid loss control additives. In addition, graft copolymers comprising a backbone of lignin or lignite and pendant groups comprising at least one member selected from the group consisting of 2-acrylamido-2-methylpropanesulfonic acid, acrylonitrile, and N,N-dimethylacrylamide have been used as fluid loss control additives. These fluid loss control additives, however, may not provide a desired level of fluid loss control at high temperatures (e.g., at least about 500° F. (260° C.)) and/or may be exhibit undesirable properties (e.g., dispersive).

SUMMARY

The present invention relates to cementing operations. More particularly, the present invention relates to cement compositions comprising a graft copolymer fluid loss control additive, the graft copolymer fluid loss control additive comprising a backbone of a humic acid salt and at least four grafted monomers selected from the group consisting of 2-acrylamido-2-methylpropanesulfonic acid, acrylamide, acrylic acid, vinylphosphonic acid, diallyldimethylammonium chloride, and salts thereof.

An exemplary embodiment of the present invention provides a well cement composition. The well cement composition comprises water, a cement, and a fluid loss control additive, the fluid loss control additive comprising a graft copolymer. The graft copolymer comprises a backbone and at least four grafted monomers selected from the group consisting of 2-acrylamido-2-methylpropanesulfonic acid, acrylamide, acrylic acid, vinylphosphonic acid, diallyldimethylammonium chloride, and salts thereof. The backbone of the graft copolymer comprises a humic acid salt.

Another exemplary embodiment of the present invention provides a well cement composition. The well cement composition comprises water, a hydraulic cement, and a fluid loss control additive, the fluid loss control additive comprising a graft copolymer. The graft copolymer comprises a backbone and grafted monomers. The backbone comprises a humic acid salt, and the grafted monomers comprise 2-acrylamido-2-methylpropanesulfonic acid salt, acrylamide, acrylic acid salt, and diallyldimethylammonium chloride.

Another exemplary embodiment of the present invention provides a well cement composition. The well cement composition comprises water, a hydraulic cement, and a fluid loss control additive, the fluid loss control additive comprising a graft copolymer. The graft copolymer comprises a backbone and grafted monomers. The backbone comprises humic acid salt, and the grafted monomers comprise 2-acrylamido-2-methylpropanesulfonic acid salt, acrylamide, vinylphosphonic acid salt, and diallyldimethylammonium chloride.

The features and advantages of the present invention will be apparent to those skilled in the art upon reading the following description of specific embodiments.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention relates to cementing operations. More particularly, the present invention relates to cement compositions comprising a graft copolymer fluid loss control additive, the graft copolymer fluid loss control additive comprising a backbone of a humic acid salt and at least four grafted monomers selected from the group consisting of 2-acrylamido-2-methylpropanesulfonic acid (AMPS® acid), acrylamide, acrylic acid, vinylphosphonic acid (VPA), diallyldimethylammonium chloride (DADMAC), and salts thereof. While these graft copolymers are effective at reducing fluid loss from cement compositions in a variety of cementing applications, they should be particularly useful for reducing fluid loss in high temperature applications, for example, in wells having a bottom hole circulating temperature (BHCT) of at least about 500° F. (260° C.).

An exemplary embodiment of the cement compositions of the present invention comprises water, cement, and a fluid loss control additive comprising a graft copolymer, the graft copolymer comprising a backbone of a humic acid salt and at least four grafted monomers selected from the group consisting of AMPS® acid, acrylamide, acrylic acid, VPA, DADMAC, and salts thereof. Those of ordinary skill in the art will appreciate that the exemplary cement compositions generally should have a density suitable for a particular application. By way of example, exemplary embodiments of the cement compositions may have a density in the range of from about 8 pounds per gallon (“ppg”) to about 21 ppg.

The water used in exemplary embodiments of the cement compositions of the present invention may be freshwater or saltwater (e.g., water containing one or more salts dissolved therein, seawater, brines, saturated saltwater, etc.). Generally, the water may be from any source, provided that it does not contain an excess of compounds that undesirably affect the cement composition. In general, the water may be present in an amount sufficient to form a pumpable slurry. In exemplary embodiments, the water may be present in the cement compositions in an amount in the range of from about 33% to about 200% by weight of the cement (“bwoc”). In exemplary embodiments, the water may be present in an amount in the range of from about 35% to about 70% bwoc.

Exemplary embodiments of the cement compositions of the present invention comprise a cement. Any of a variety of cements suitable for use in subterranean cementing operations may be used in accordance with exemplary embodiments of the present invention. Suitable examples include hydraulic cements that comprise calcium, aluminum, silicon, oxygen, and/or sulfur, which set and harden by reaction with water. Such hydraulic cements, include, but are not limited to, Portland cements, pozzolana cements, gypsum cements, high alumina content cements, slag cements, and silica cements, and combinations thereof. In certain embodiments, the hydraulic cement may comprise a Portland cement. In some embodiments, the Portland cements that are suited for use in exemplary embodiments of the present invention are classified as Class A, C, H, and G cements according to American Petroleum Institute, API Specification for Materials and Testing for Well Cements, API Specification 10, Fifth Ed., Jul. 1, 1990.

In addition, exemplary embodiments of the cement compositions of the present invention comprise a fluid loss control additive. As previously mentioned, such fluid loss control additives comprise a graft copolymer that comprises a backbone of a humic acid salt, wherein the humic acid salt is grafted with at least four monomers selected from the group consisting of AMPS® acid, acrylamide, acrylic acid, VPA, DADMAC, and salts thereof. In general, these graft copolymers are effective at reducing fluid loss from cement compositions in a variety of cementing operations. By way of example, these graft copolymers are particularly suitable for use as fluid loss control additives in high temperature applications, for example, in wells with a BHCT of at least about 500° F. (260° C.).

Generally, the backbone of the graft copolymer comprises a humic acid salt. In general, humic acids are compounds that may result from the decomposition of organic matter. By way of example, humic acid may comprise allomelanins found in soils, coals, and peat, resulting from the decomposition of organic matter, such as dead plants. In addition, in exemplary embodiments, the backbone may further comprise polyvinyl alcohol, polyethylene oxide, polypropylene oxide, polyethyleneimine, and combinations thereof. Those of ordinary skill in the art will appreciate that the humic acid may be treated with a caustic (for example, potassium hydroxide, sodium hydroxide, or ammonium hydroxide) to solubilize the humic acid in water. By way of example, treatment of the humic acid with a caustic solution generally may dissolve or disperse a portion of the humic acid into the solution. Such solution may then be concentrated to increase the humic acid content or may be used directly in the polymerization.

The graft copolymer may comprise a sufficient amount of the humic acid salt backbone to provide a desirable level of fluid loss control. In exemplary embodiments, the backbone may be present in the graft copolymer in an amount in the range of from about 5% to about 95% by weight of the graft copolymer on a dry basis. In exemplary embodiments, the backbone may be present in the graft copolymer in an amount in the range of from about 10% to about 50% by weight of the graft copolymer on a dry basis. In exemplary embodiments, the backbone may be present in the graft copolymer in an amount in the range of from about 10% to about 20% by weight of the graft copolymer on a dry basis.

As previously mentioned, the humic acid salt backbone of the graft copolymer is grafted with at least four monomers selected from the group consisting of AMPS® acid, acrylamide, acrylic acid, VPA, DADMAC, and salts thereof. In general, the grafted monomers are present in pendant groups attached to the humic acid salt backbone in a random nature. By way of example, each pendant group attached to the humic acid salt backbone may contain one or more of the monomers in a random nature. Without being limited by theory, it is believed that the various monomers (e.g., at least four of AMPS® acid, acrylamide, acrylic acid, VPA, and DADMAC) attach at free radicals created in the humic acid salt backbone. In exemplary embodiments, at least four monomers selected from the group consisting of the AMPS® acid, acrylamide, acrylic acid, VPA, DADMAC, and salts thereof may be admixed with the humic acid salt prior to initiation of the polymerization reaction.

Generally, the graft copolymer may comprise a sufficient amount of these grafted monomers to provide a desirable level of fluid loss control. In exemplary embodiments, the grafted monomers may be present in the graft copolymer in an amount in the range of from about 5% to about 95% by weight of the graft copolymer on a dry basis. In exemplary embodiments, the grafted monomers may be present in the graft copolymer in an amount in the range of from about 50% to about 90% by weight of the graft copolymer on a dry basis. In exemplary embodiments, the grafted monomers may be present in the graft copolymer in an amount in the range of from about 80% to about 90% by weight of the graft copolymer on a dry basis.

An exemplary fluid loss control additive comprises a humic acid salt backbone grafted with AMPS® acid salt, acrylamide, acrylic acid salt, and DADMAC. Each of these grafted monomers may be present in the exemplary graft copolymer in a variety of different mole ratios. In an exemplary embodiment, the exemplary graft copolymer may have an AMPS® acid salt-to-acrylamide mole ratio in the range of from about 1:1 to about 4:1. In an exemplary embodiment, the exemplary graft copolymer may have an AMPS® acid salt-to-acrylic acid salt mole ratio in the range of from about 1:1 to about 10:1. In an exemplary embodiment, the exemplary graft copolymer may have an AMPS® acid salt-to-DADMAC mole ratio in the range of from about 1:1 to about 10:1. In an exemplary embodiment, the exemplary graft copolymer may have an AMPS® acid salt-to-acrylamide-to-acrylic acid salt-to-DADMAC mole ratio in the range of from about 4:3:2:1 to about 4:1:0.5:0.1. In another exemplary embodiment, the exemplary graft copolymer may have an AMPS® acid salt-to-acrylamide-to-acrylic acid salt-to-DADMAC mole ratio in the range of from about 4:3:1:1 to about 4:2:1:0.25.

Another exemplary fluid loss control additive comprises a humic acid salt backbone grafted with AMPS® acid salt, acrylamide, VPA salt, DADMAC. Each of these grafted monomers may be present in the exemplary graft copolymer in a variety of different mole ratios. In an exemplary embodiment, the exemplary graft copolymer may have an AMPS® acid salt-to-acrylamide mole ratio in the range of from about 1:1 to about 4:1. In an exemplary embodiment, the exemplary graft copolymer may have an AMPS® acid salt-to-VPA salt mole ratio in the range of from about 2:1 to about 10:1. In an exemplary embodiment, the exemplary graft copolymer may have an AMPS® acid salt-to-DADMAC mole ratio in the range of from about 1:1 to about 10:1. In an exemplary embodiment, the exemplary graft copolymer may have an AMPS® acid salt-to-acrylamide-to-VPA salt-to-DADMAC mole ratio in the range of from about 4:4:1:2 to about 4:1:0.1:0.1. In another exemplary embodiment, the exemplary graft copolymer may have an AMPS® acid salt-to-acrylamide-to-VPA salt-to-DADMAC mole ratio in the range of from about 4:4:1:2 to about 4:2:1:0.25.

Those of ordinary skill in the art will appreciate that the amount of each of the grafted monomers present in the graft copolymer is dependent upon a number of factors, including cost, the temperature of use, the desired molecular weight of the graft copolymer, dispersive properties, and desired amount of fluid loss control.

Those of ordinary skill in the art will appreciate that suitable graft copolymers may be in an acid form or in a salt form. As will be appreciated, a variety of salts of the graft copolymers may be prepared, for example, by neutralizing the acid form of the AMPS® acid, acrylic acid monomer, and/or VPA monomer with an alkaline agent. By way of example, suitable alkaline agents may include sodium hydroxide, ammonium hydroxide or the like. This neutralization may occur, for example, either prior to or after the polymerization of the graft copolymer. By way of example, a salt of AMPS® acid (such as the sodium salt of AMPS® acid) may be used in the polymerization of the graft copolymer. In addition, neutralization of the monomers may occur, for example, due to calcium ions present in the exemplary cement compositions.

The fluid loss control additive should be included in exemplary embodiments of the cement compositions of the present invention in an amount sufficient to provide a desired level of fluid loss control. In exemplary embodiments, the fluid loss control additive may be present in exemplary embodiments of the cement compositions in an amount in the range of from about 0.1% to about 5% bwoc. In exemplary embodiments, the fluid loss control additive may be present in exemplary embodiments of the cement compositions in an amount in the range of from about 0.2% to about 3% bwoc.

Exemplary embodiments of the graft copolymers used as fluid loss control additives in accordance with the present invention may be prepared using any suitable polymerization technique. By way of example, suitable graft copolymers may be prepared using any of a variety of free radical polymerization techniques. The initiators employed in these free radical techniques to produce exemplary embodiments of the graft copolymer may generally comprise a redox initiator capable of generating a free radical in the humic acid which will initiate polymerization by reacting with the monomers. While various ceric salts and other compounds may be suitable to effect the free radical initiation, an ammonium persulfate-sodium bisulfite redox system may be employed for the preparation of exemplary embodiments of the humic acid graft copolymers. In certain embodiments, water may be employed as the solvent in the polymerization reactions. Those of ordinary skill in the art, however, will appreciate that other solvents capable of solubilizing the reactants as desired also may be utilized.

Other additives suitable for use in subterranean cementing operations also may be added to exemplary embodiments of the cement compositions. Examples of such additives include dispersing agents, set retarding agents, accelerants, defoaming agents, lost circulation materials, salts, fly ash, fiber, strength retrogression additives, weighting agents, vitrified shale, lightweight additives (e.g., bentonite, gilsonite, glass spheres, etc.), and combinations thereof. A person having ordinary skill in the art, with the benefit of this disclosure, will readily be able to determine the type and amount of additive useful for a particular application and desired result.

As will be appreciated by those of ordinary skill in the art, exemplary embodiments of the cement compositions of the present invention may be used in a variety of subterranean applications, including primary and remedial cementing. Exemplary embodiments of the cement compositions may be introduced into a subterranean formation and allowed to set therein. Exemplary embodiments of the cement compositions may comprise water, cement, and a fluid loss control additive comprising a graft copolymer, the graft copolymer comprising a humic acid salt backbone and at least four grafted monomers selected from the group consisting of AMPS® acid, acrylamide, acrylic acid, VPA, DADMAC, and salts thereof. By way of example, in exemplary primary cementing embodiments, a cement composition may be introduced into a space between a subterranean formation and a pipe string located in the subterranean formation. The cement composition may be allowed to set to form a hardened mass in the space between the subterranean formation and the pipe string. In addition, in exemplary remedial cementing embodiments, a cement composition may used, for example, in squeeze cementing operations or in the placement of cement plugs.

To facilitate a better understanding of the present technique, the following examples of some specific exemplary embodiments are given. In no way should the following examples be read to limit, or to define, the scope of the invention.

EXAMPLE 1

Copolymers comprising a humic acid salt backbone grafted with at least four monomers selected from AMPS® acid, acrylamide, acrylic acid, VPA, DADMAC, and salts thereof were prepared in accordance with the following procedure. First, sodium humate, water, a defoamer, and ethylenediaminetetraacetic acid (EDTA) were added to a reactor vessel. To this vessel, a 58% by weight solution of the sodium salt of AMPS® acid, a 48% by weight solution of acrylamide, acrylic acid (or VPA as set forth in the table below), and DADMAC were then added. This mixture was heated and held at a temperature of about 158° F. (70° C.) for one hour while purging with nitrogen. After one hour, ammonium persulfate was added to initiate polymerization. This mixture was held at a temperature of about 158° F. (70° C.) for two hours. After two hours, sodium metabisulfite was added, and the mixture was allowed to cool. When the mixture reached room temperature, a 50% solution of sodium hydroxide was added to adjust the pH in the range of from about 7 to about 8. The weight percent of each component utilized in the copolymer's preparation is listed in the table below.

TABLE 1 Formulation #1 Formulation #2 Component (% by wt) (% by wt) Sodium Humate 4.7 4.7 Water 42.84 41.01 NaOH 1.69 1.28 (50% solution) Defoamer 0.01 0.01 NaAMPS 31.48 31.48 (58% solution) Acrylamide 5.92 5.92 (48% solution) Acrylic Acid 1.44 — VPA — 1.08 DADMAC 5.22 7.82 (62% solution) NaEDTA 0.1 0.1 Ammonium 3.3 3.3 Persulfate (27% solution) Sodium 3.3 3.3 Metabisulfite (27% solution)

EXAMPLE 2

The following series of tests were performed to compare the performance of exemplary embodiments of the fluid loss control additives of the present invention with HALAD®-413 fluid loss additive, which is described in U.S. Pat. No. 4,676,317. Accordingly, sample cement compositions were prepared that comprised Portland Class H cement, HR®-12 retarder (0.6% bwoc), a free water control additive (0.1% bwoc), a fluid loss control additive 1% bwoc), and sufficient water to provide a density of 16.4 ppg. HR®-12 retarder is a cement set retarder available from Halliburton Energy Services, Inc. In addition, the free water control additive used was hydroxyethyl cellulose, available from Halliburton Energy Services, Inc.

With respect to the fluid loss control additives, Sample No. 1 included HALAD®-413 fluid loss additive as the fluid loss control additive. Sample Nos. 2-9 comprised the exemplary embodiments of the fluid loss control additive of the present invention having a humic acid concentration of between 12% to 16% by weight and mole ratios of the grafted monomers as set forth in the table below.

TABLE 2 Mole Ratio Acrylic Polymer NaAMPS Acrylamide Acid VPA DADMAC Humic Acid 0.4 0.2 0.1 — 0.1 Graft Copolymer 1 Humic Acid 0.4 0.25 0.1 — 0.05 Graft Copolymer 1A Humic Acid 0.4 0.275 0.1 — 0.025 Graft Copolymer 1B Humic Acid 0.3 0.375 0.1 — 0.025 Graft Copolymer 1C Humic Acid 0.3 0.4125 0.05 — 0.0375 Graft Copolymer 1D Humic Acid 0.4 0.2 — 0.05 0.15 Graft Copolymer 2 Humic Acid 0.4 0.275 — 0.05 0.075 Graft Copolymer 2A Humic Acid 0.4 0.3125 — 0.05 0.375 Graft Copolymer 2B

After preparation, each sample cement composition was poured into a pre-heated cell with a 325-mesh screen, and a fluid loss test was performed for 30 minutes at 1,000 psi and the temperature listed in the table below. The fluid loss tests were performed in accordance with API RP 10B, Recommended Practices for Testing Well Cements. Additionally, the rheological properties of the sample cement compositions were also determined using a Fann® Model 35 viscometer at the temperature listed in the table below, in accordance with the above-mentioned APP RP 10B. The results of these tests are given in the table below.

TABLE 3 Rheology Tests: Viscometer Readings Sample Density Fluid Loss Control Temp. Fluid Loss 300 200 100 60 30 6 3 600 No. (ppg) Additive (1% bwoc) (° F.) (cc/30 min.) RPM RPM RPM RPM RPM RPM RPM RPM 1 16.4 HALAD ®-413 80 n/d 300+ 300+ 250 172 101 29 16 300+ 180 30 300+ 232 127  80 43 10 5 300+ 2 16.4 Humic Acid 80 n/d 300+ 300+ 270 183 107 29 16 300+ Graft 180 24 300+ 270 143  88 46 9 5 300+ Copolymer 1 3 16.4 HumicAcid 80 n/d 300+ 300+ 279 189 110 30 17 300+ Graft 180 20 300+ 244 124  77 40 9 7 300+ Copolymer 1A 4 16.4 Humic Acid 80 n/d 300+ 300+ 296 200 116 32 19 300+ Graft 180 20 300+ 243 128  82 46 12 7 300+ Copolymer 1B 5 16.4 Humic Acid 80 n/d 300+ 300+ 300+ 204 120 34 19 300+ Graft 180 22 300+ 282 153  95 52 12 7 300+ Copolymer 1C 6 16.4 Humic Acid 80 n/d 300+ 300+ 224 144 80 19 10 300+ Graft 180 18 300+ 250 129  78 40 9 4 300+ Copolymer 1D 7 16.4 Humic Acid 80 n/d 300+ 300+ 300+ 300+ 241 63 32 300+ Graft 180 20 300+ 300+ 268 164 81 15 7 300+ Copolymer 2 8 16.4 Humic Acid 80 n/d 300+ 300+ 239 157 89 23 13 300+ Graft 180 19 300+ 216 115  71 37 8 5 300+ Copolymer 2A 9 16.4 Humic Acid 80 n/d 300+ 300+ 239 162 93 24 14 300+ Graft 180 19 300+ 250 132  81 42 9 5 300+ Copolymer 2B

Thus, Example 2 indicates, inter alia, that the use of exemplary embodiments of the fluid loss control additives of the present invention provides improved fluid loss control and more desirable rheology as compared to HALAD®-413 fluid loss additive.

EXAMPLE 3

Additional tests were performed to compare the performance of exemplary embodiments of the fluid loss control additives of the present invention with HALAD®-413 fluid loss additive at high temperatures. Accordingly, sample cement compositions were prepared that comprised Portland Class H cement, HR®-12 retarder, a free water control additive, SSA-1 strength stabilizing agent, a fluid loss control additive, and sufficient water to provide a density of 16.4 ppg. The free water control additive used was hydroxyethyl cellulose, available from Halliburton Energy Services, Inc. Sample No. 10 did not include the free water control additive. SSA-1 is a strength stabilizing agent (crystalline silica) available from Halliburton Energy Services, Inc. The specific fluid loss control additive included in each sample is set forth in the table below.

After preparation, fluid loss tests were performed at 1,000 psi and the temperature listed in the table below. For this series of tests, the fluid loss was determined using a stirring fluid loss apparatus rated for testing at temperatures up to 400° F. and pressures up to 2,000 psi. The stirring fluid-loss apparatus is described in API RP10B-2/ISO 10426-2 in section 10. For these fluid loss tests, the sample was stirred in the test cell while the temperature was increased to the test temperature. After the sample reached the test temperature, rotation of the paddle inside the cell was stopped, the cell was rotated to place the sample against the filter medium (a 325-mesh sieve), and the fluid loss was determined. The filtrate from the sample flowed into a filtrate collection chamber that had back pressure applied to it in order to keep it from boiling for any tests containing salt. The results of these tests are given in the table below.

TABLE 4 HR ® Fluid Loss Fluid Sample Density Retarder FWCA SSA-1 Control Additive Temp. Loss No. (ppg) (% bwoc) (% bwoc) (% bwoc) (1% bwoc) (° F.) (cc/30 min.) 10 16.4 0.6 — 35 Humic Acid Graft 250 28 Copolymer 2 11 16.4 1.5 0.1 35 Humic Acid Graft 300 60 Copolymer 2 12 16.4 1.5 0.1 35 Humic Acid Graft 350 44 Copolymer 1B 13 16.4 1.5 0.1 35 Humic Acid Graft 350 32 Copolymer 2B 14 16.4 2 0.1 35 HALAD ®-413 400 82 15 16.4 2 0.1 35 Humic Acid Graft 400 68 Copolymer 1B 16 16.4 2 0.1 35 Humic Acid Graft 400 96 Copolymer 1C 17 16.4 2 0.1 35 Humic Acid Graft 400 94 Copolymer 1D 18 16.4 2 0.1 35 Humic Acid Graft 400 68 Copolymer 2B

Thus, Example 3 indicates, inter alia, that the use of exemplary embodiments of the fluid loss control additives of the present invention may provide improved fluid loss control as to HALAD®-413 fluid loss additive.

EXAMPLE 4

Additional tests were performed to evaluate the performance of exemplary embodiments of the fluid loss control additives of the present invention. Accordingly, sample cement compositions were prepared that comprised Portland Class G cement, SSA-1 strength stabilizing agent (35% bwoc), HR®-5 retarder (0.3% bwoc), Microbond™ expanding additive (0.1% bwoc), a fluid loss control additive (1% bwoc), Silicalite® cement additive (10% bwoc), bentonite (2% bwoc), and sufficient water to provide a density of 15.2 ppg. HR®-5 retarder is a cement set retarder available from Halliburton Energy Services, Inc. Microbond™ expanding additive is a cement additive available from Halliburton Energy Services, Inc. Silicalite® cement additive is a finely divided, high surface area amorphous silica available from Halliburton Energy Services, Inc. The specific fluid loss control additive included in each sample is set forth in the table below.

After preparation, each sample cement composition was poured into a pre-heated cell with a 325-mesh screen, and a fluid loss test was performed for 30 minutes at 1,000 psi and the temperature listed in the table below. The fluid loss tests were performed in accordance with API RP 10B, Recommended Practices for Testing Well Cements. Additionally, the rheological properties of the sample cement compositions were also determined using a Fann® Model 35 viscometer at the temperature listed in the table below, in accordance with the above-mentioned APP RP 10B. The results of these tests are given in the table below.

TABLE 5 Rheology Tests: Viscometer Readings Sample Density Fluid-Loss-Control Temp. Fluid Loss 300 200 100 60 30 6 3 600 No. (ppg) Additive (1% bwoc) (° F.) (cc/30 min.) RPM RPM RPM RPM RPM RPM RPM RPM 19 15.2 Humic Acid Graft 80 n/d 154 115 67 46 29 10 7 263 Copolymer 1B 180 62 160 117 70 48 30 12 9 215 20 15.2 Humic Acid Graft 80 n/d 132 94 51 33 18 5 3 227 Copolymer 2B 180 44 135 84 46 30 18 6 4 180

Thus, Example 4 indicates, inter alia, that the use of exemplary embodiments of the fluid loss control additives of the present invention may provide desirable fluid loss control and rheology.

EXAMPLE 5

Additional tests were performed to evaluate the performance of exemplary embodiments of the fluid loss control additives of the present invention. Accordingly, sample cement compositions were prepared that comprised Portland Class H cement, SSA-1 strength stabilizing agent (35% bwoc), SCR®-100 retarder (0.75% bwoc), CFR™-3 cement dispersant (0.75% bwoc), HR®-25 retarder (0.1% bwoc), a fluid loss control additive (1% bwoc), and sufficient water to provide a density of 17.5. HR®-25 retarder is a cement set retarder available from Halliburton Energy Services, Inc. SCR®-100 retarder is a cement set retarder available from Halliburton Energy Services, Inc. CFR™-3 cement dispersant is a cement dispersant available from Halliburton Energy Services, Inc. The specific fluid loss control additive included in each sample is set forth in the table below.

After preparation, each sample cement composition was poured into a pre-heated cell with a 325-mesh screen, and a fluid loss test was performed for 30 minutes at 1,000 psi and the temperature listed in the table below. The fluid loss tests were performed in accordance with API RP 10B, Recommended Practices for Testing Well Cements. The results of these tests are given in the table below.

TABLE 6 Fluid Loss Howco Fluid Control Consistometer Loss Sample Density Additive Temp. Reading (cc/ No. (ppg) (1% bwoc) (° F.) 0 min 20 min 30 min.) 21 17.5 HALAD ®- 180 18 14 100 413 22 17.5 Humic Acid 180 17 13 58 Graft Copolymer 2B

Thus, Example 5 indicates, inter alia, that the use of exemplary embodiments of the fluid loss control additives of the present invention provides improved fluid loss control as compared to HALAD®-413 fluid loss additive.

EXAMPLE 6

Compressive strength and thickening time tests were performed to compare the performance of sample cement compositions that comprised exemplary embodiments of the fluid loss control additives of the present invention with HALAD®-413 fluid loss additive. Accordingly, sample cement compositions were prepared that comprised Portland Class H cement, SSA-1 strength stabilizing agent (35% bwoc), a free water control additive (0.1% bwoc), HR®-12 retarder, a fluid loss control additive (1% bwoc), and sufficient water to provide a density of 16.4 ppg. The free water control additive used was hydroxyethyl cellulose, available from Halliburton Energy Services, Inc. The specific fluid loss control additive included in each sample is set forth in the table below.

After preparation, the sample cement compositions were subjected to 48-hour compressive strength tests and thickening time tests at the temperature listed in the table below, in accordance with API RP10B, Recommended Practices for Testing Well Cements. The results of these tests are set forth in the table below.

TABLE 7 Fluid Loss Thickening Control Time to 70 Comp. Sample Density Additive Temp. BC Strength No. (ppg) (1% bwoc) (° F.) (Hr:Min) (psi) 23 16.4 HALAD ®-413 250 6:46 3,963 24 16.4 Humic Acid 250 5:56 4,050 Graft Copolymer 1B 25 16.4 Humic Acid 250 9:46 4,505 Graft Copolymer 2B 26 16.4 HALAD ®-413 325 4:46 n/d 27 16.4 Humic Acid 325 3:42 n/d Graft Copolymer 1B 28 16.4 Humic Acid 325 3:46 n/d Graft Copolymer 2B

Thus, Example 6 indicates, inter alia, that the use of exemplary embodiments of the fluid loss control additives of the present invention may provide desirable thickening times and compressive strengths.

Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be susceptible to various modifications and alternative forms. However, it should be understood that the invention is not intended to be limited to the particular embodiments disclosed. Rather, the present invention is to cover all modifications, equivalents, and alternatives falling within the scope and spirit of the present invention as defined by the following appended claims. 

1. A well cement composition comprising: water; a cement; and a fluid loss control additive comprising a graft copolymer, the graft copolymer comprising a backbone and at least four grafted monomers selected from the group consisting of 2-acrylamido-2-methylpropanesulfonic acid, acrylamide, acrylic acid, vinylphosphonic acid, diallyldimethylammonium chloride, and salts thereof, the backbone comprising a humic acid salt.
 2. The well cement composition of claim 1, wherein the water is present in an amount in the range of from about 35% to about 70% by weight of the cement.
 3. The well cement composition of claim 1, wherein the water is selected from the group consisting of freshwater and saltwater.
 4. The well cement composition of claim 1, wherein the cement comprises a hydraulic cement.
 5. The well cement composition of claim 1, wherein the cement comprises a hydraulic cement selected from the group consisting of a Portland cement, a pozzolana cement, a gypsum cement, a high alumina content cement, a slag cement, a silica cement, and combinations thereof.
 6. The well cement composition of claim 1, wherein the graft copolymer comprises the backbone in an amount in the range of from about 10% to about 50% by weight of the graft copolymer on a dry basis, and wherein the graft copolymer comprises the at least four grafted monomers in an amount in the range of from about 50% to about 90% by weight of the graft copolymer on a dry basis.
 7. The well cement composition of claim 1, wherein the backbone comprises a monomer selected from the group consisting of polyvinyl alcohol, polyethylene oxide, polypropylene oxide, polyethyleneimine, and combinations thereof.
 8. The well cement composition of claim 1, wherein the grafted monomers comprise the 2-acrylamido-2-methylpropanesulfonic acid salt, the acrylamide, the acrylic acid salt, and the diallyldimethylammonium chloride.
 9. The well cement composition of claim 1, wherein the graft copolymer has an 2-acrylamido-2-methylpropanesulfonic acid salt-to-acrylamide-to-acrylic acid salt-to-diallyldimethylammonium chloride mole ratio in the range of from about 4:3:2:1 to about 4:1:0.5:0.1.
 10. The well cement composition of claim 1, wherein the graft copolymer has an 2-acrylamido-2-methylpropanesulfonic acid salt-to-acrylamide-to-vinylphosphonic acid salt-to-diallyldimethylammonium chloride mole ratio in the range of from about 4:4:1:2 to about 4:1:0.1:0.1.
 11. The well cement composition of claim 1, the backbone is present in the graft copolymer in an amount in the range of from about 10% to about 20% by weight on a dry basis.
 12. The well cement composition of claim 1, wherein the fluid loss control additive is present in the cement composition in an amount in the range of from about 0.1% to about 5% by weight of the cement.
 13. The well cement composition of claim 1, wherein the well cement composition comprises an additive selected from the group consisting of a dispersing agent, a set retarding agent, an accelerant, a defoaming agent, a lost circulation material, a salt, fly ash, fiber, a strength retrogression additive, a weighting agent, vitrified shale, a lightweight additive, and combinations thereof.
 14. The well cement composition of claim 1, wherein the well cement composition comprises crystalline silica.
 15. A well cement composition comprising: water; a hydraulic cement; and a fluid loss control additive comprising a graft copolymer, the graft copolymer comprising a backbone and grafted monomers, the backbone comprising a humic acid salt, the grafted monomers comprising 2-acrylamido-2-methylpropanesulfonic acid salt, acrylamide, acrylic acid salt, and diallyldimethylammonium chloride.
 16. The well cement composition of claim 15, wherein the graft copolymer comprises the grafted monomers in an 2-acrylamido-2-methylpropanesulfonic acid salt-to-acrylamide-to-acrylic acid salt-to-diallyldimethylammonium chloride mole ratio in the range of from about 4:3:2:1 to about 4:1:0.5:0.1.
 17. The well cement composition of claim 15, wherein the graft copolymer comprises the backbone in an amount in the range of from about 10% to about 50% by weight of the graft copolymer on a dry basis, and wherein the graft copolymer comprises the at least four grafted monomers in an amount in the range of from about 50% to about 90% by weight of the graft copolymer on a dry basis.
 18. A well cement composition comprising: water; a hydraulic cement; and a fluid loss control additive comprising a graft copolymer, the graft copolymer comprising a backbone and grafted monomers, the backbone comprising a humic acid salt, the grafted monomers comprising 2-acrylamido-2-methylpropanesulfonic acid salt, acrylamide, vinylphosphonic acid salt, and diallyldimethylammonium chloride.
 19. The well cement composition of claim 18, wherein the graft copolymer has an 2-acrylamido-2-methylpropanesulfonic acid salt-to-acrylamide-to-vinylphosphonic acid salt-to-diallyldimethylammonium chloride mole ratio in the range of from about 4:4:1:2 to about 4:1:0.1:0.1.
 20. The well cement composition of claim 18, wherein the graft copolymer comprises the backbone in an amount in the range of from about 10% to about 50% by weight of the graft copolymer on a dry basis, and wherein the graft copolymer comprises the at least four grafted monomers in an amount in the range of from about 50% to about 90% by weight of the graft copolymer on a dry basis. 